If your warlock's patron is an insane fey, or a necromancer fiend, would the warlock's archetype be Archfey or Great Old One in the former case, Fiend or Undying in the second case? Would it have to be Archfey and Fiend because that's the creature type of the patron, or would I be free to choose Great Old One or Undying due to the way the patron is characterised? For example, Orcus is a Demon Lord and a necromancer, so could he serve as an Undying patron, or only a Fiend patron?
I ask this because I cannot think of any patrons that would suit the Seeker, being interested in seeking knowledge and travelling the Astral Plane (it doesn't make sense to me that patrons can be deities, since that's what a cleric is, and the Unearthed Arcana article that introduced the Seeker patron only mentions suitable deities).
The DMG (pg. 48) says about the Astral Plane:
Celestials, fiends, and mortal explorers often scour the Astral Plane for color pools leading to desired destinations. Characters who linger for too long in the Astral Plane might have an encounter with one or more wandering angels, demons, devils, night hags, yugoloths, or other planar travelers.
So that means, besides Githyanki, which are the only other creature mentioned on that page (I'm ignoring the "mortal explorers" here because they don't impact my main question), only angels and various fiends go to the astral plane, meaning that those patrons would have to be Fiend or Celestial warlock patrons... or would they? Hence my main question.
Note that what creatures (rather than gods) would make a decent Seeker patron is outside the scope of this question; I want to try to keep this focused on something answerable in the RPG.SE format.